1. Field of the Invention
This invention relates to compositions free from mineral oil for freeing jammed drill pipes, and to a method of using such compositions.
2. Statement of Related Art
In drilling for oil or natural gas, a well fluid is normally fed to the well through the drill pipes. The function of a well fluid is not only to cool and lubricate the drill pipes, but also to apply a pressure to the formations surrounding the well and hence to equalize the mechanical pressure of the formation on the well. In addition, well fluids of this type are thickened to such an extent that they are capable of discharging the wet drill cuttings through the annular drill space surrounding the drill pipes.
The specific gravity of a well fluid is normally adjusted in such a way that the pressure of the rock formations on the well is exceeded. As a result, liquid components of the well fluid are forced into the formations surrounding the well while insoluble components settle on the walls of the well in the form of stabilizing, so-called "filter cake".
It is not always possible, particularly with relatively deep wells, to prevent the path followed by the drill pipes (and hence by the well itself) from deviating from the vertical. This means that, in the event of a such a "kink", the rotating drill pipes come into contact with the filter cake described above and later with the wall of the well and gradually become buried therein. This happens in particular when the pipe connections, which are somewhat larger in diameter than the drill pipes themselves, come into contact with the kinks. The most frequent cause of jamming (sticking) in the case of drill pipes is differential jamming. Differential jamming can always occur when the rotation of the drill pipes is stopped. If the drill pipes then come into contact with the filter cake adhering to the wall of the well, they are forced into the wall of the well by the hydrostatic pressure of the column of mud. The contact surface is then isolated from the mud pressure. If, as is normally the case, the mud pressure is greater than the formation pressure, the contact surface is held fast to the wall by suction forces. The suction force increases with the thickness and compressibility of the filter cake. With time, even relatively large sections of the drill pipe can become held fast. Accordingly, prompt treatment is necessary.
Prompt freeing of the jammed drill pipes is also necessary on account of the time lost during drilling and the immense costs which this involves.
In practice, the use of certain compositions normally known to those skilled in the art as spotting fluids has proved to be a convenient method of freeing jammed drill pipes. Spotting fluids are required to have a good lubricating effect and to ensure good oil wettability of the surfaces, which means not only the surfaces of the drill pipes, but also the walls of the well coming into contact with the drill pipes and the filter cake formed by the well fluid.
To be effective in freeing the jammed drill pipes, a spotting fluid can be applied at exactly that point of the well at which the drill pipes have jammed. This is done in the same way in which the well fluid is introduced into the well. Instead of the well fluid, the spotting fluid is forced through the interior of the drill pipes to the bit, leaves the bit and slowly ascends in the well to the point at which the problem is localized. It is therefore of crucial importance to be able exactly to adjust the specific gravity of the well fluid previously used. Only in this way is it possible to keep such a lubricity-enhancing composition for freeing jammed drill pipes in place for a relatively long time at that point of the well which is causing the problem so that the fluid has sufficient time in which to act.
Since the prompt use of a spotting fluid is extremely important for the reasons explained above, it is standard practice at the drill site to store concentrates which may rapidly be adjusted to the required density and consistency by addition of barite or other weighting materials and, optionally, other additives and water. This saves costs and unnecessary down time.
Spotting fluid compositions for freeing jammed drill pipes are known from the prior art. Normally, spotting fluids consist mainly of hydrocarbon mixtures based primarily on diesel oils. In recent years, mineral oils low in aromatic fractions have also been used as the principal constituent of spotting fluids. Emulsifiers and surfactants are then normally added to the spotting fluids. Spotting fluids of this type based on diesel oil or paraffin oil are described, for example, in European Patent document No. EP-A 200 466 (86 303 057) and in U.S. Pat. Nos. 4,427,564, 4,436,638 and 4,464,269.
However, these spotting fluids and other spotting fluids known from the prior art are attended by major disadvantages. Both during their action at the point of application and during their discharge after the drill pipes have been freed, spotting fluids of the type in question may come into contact with ground soil formations, seep into the ground and can cause long-term, almost irreversible damage if care is not taken. In addition, the oil-based spotting fluid, which mixes with the water-based well fluid during its discharge from the well, has to be dumped and treated separately from the large quantity of residual water-based well fluid in order to avoid the environmental damage mentioned above. Because the spotting fluid mixes with the well fluid during discharge, a certain quantity of the water-based well fluid also has to be discarded and separately treated. As a consequence, such losses of water-based well fluid result in additional costs. A further disadvantage lies in the fact that, with respect to off-shore drilling on the high seas, it is not possible to prevent part of the well fluid, and hence part of the mineral-oil-containing spotting fluids, from entering and being washed away by seawater surrounding the well. The consequence of this is that marine animals caught in the vicinity of such wells may be inedible.